NSF-EC Activity: Studies of Defects, Disorder and Phase Transitions via Diffuse Scattering

在这里，我们提出了一个联合休斯敦大学-马克斯普朗克研究所（MPI-斯图加特）计划的X射线和中子从无序合金的漫散射。 本论文主要包括三个方面的内容：a）确定了稀磁无规Ge中原子的详细的局域和长程静态位移（0.08）硅（0.92）在格勒诺布尔的ESRF上使用反常X射线散射，沿着光学模式和带内共振模式的高能量分辨率X射线或中子测量的合金晶体; B）使用零矩阵62 Ni（0.52）Pt（0.48）晶体（62 Ni，其具有负散射长度，将由MPI提供）来消除所有布拉格、热扩散和黄散射，以便评估局部有序和局部原子位移两者，用于与该合金的理论模型进行比较。 一个“正常”的镍-铂晶体也将被用来评估声子色散曲线和黄散射。 弹性和电子相互作用的理论工作将与我们的MPI同事合作进行; c）继续理论工作，以了解缺陷对相变的影响，在这种情况下，V2 H中的大块一级跃迁转换为缺陷（位错主导）“皮肤”层中的三临界跃迁的X射线观察。学生将参与这项研究。在这项工作计划中，我们将继续研究合金的详细局部结构-半导体和金属-通过扩散（即非布拉格）X射线和中子散射的应用。 合金的基本物理学取决于原子间相互作用的精确性质，包括电子和弹性。 这些相互作用可以直接探测通过散射数据和理论建模的组合，无论是通过模拟或从第一原理。 基本问题是原子的“尺寸”，即电荷转移，以及基本电子结构在局部有序、合金稳定性以及最终在相图预测中表现的方式。 统计力学是这些问题的核心，缺陷的影响，如位错，对相稳定性和相变。 我们的散射计划，结合理论建模，将解决这些问题，在一个全面的方式使用同步加速器X射线和反应堆为基础的中子。这项研究将与将在凝聚态物理学的当代前沿领域之一接受培训的学生进行。通过这一点，他们将准备进入未来几十年的科学/技术劳动力。

We propose here a joint University of Houston - Max Planck Institute (MPI-Stuttgart) program on the diffuse scattering of X-rays and neutrons from disordered alloys. There are three projects involved: a) The determination of detailed local and long-range static atomic displacements in a dilute random Ge(0.08)Si(0.92) alloy crystal using anomalous X-ray scattering at the ESRF in Grenoble, along with high energy-resolution X-ray or neutron measurements of the optic modes and in-band resonant modes; b) The use of a null matrix 62Ni(0.52)Pt(0.48) crystal (62Ni, which has a negative scattering length, will be supplied by the MPI) to eliminate all Bragg, thermal diffuse and Huang scattering in order to evaluate both the local order and local atomic displacements for comparison with theoretical models for this alloy. A "normal" Ni-Pt crystal will also be used to assess the phonon dispersion curves and Huang scattering. Companion theoretical work on the elastic and electronic interactions will be performed in collaboration with our MPI colleagues; c) Continuation of theoretical work to understand defect influences on phase transitions, in this case the X-ray observation of the conversion of a bulk first-order transition in V2H to a tricritical transition in a defective (dislocation-dominated) "skin" layer. Students will participate in this research.In this program of work we continue our studies of the detailed local structure of alloys - both semiconducting and metallic - through the application of diffuse (i.e. non-Bragg) X-ray and neutron scattering. The underlying physics of alloys depends on the precise nature of the interatomic interactions, both electronic and elastic. These interactions may be directly probed through a combination of scattering data and theoretical modeling, either via simulation or from first principles. The essential issues are the "size" of an atom, i.e. charge transfer, and the way in which the basic electronic structure is manifested in local order, alloy stability and, ultimately, in the prediction of phase diagrams. Statistical mechanics lies at the heart of these problems, as does the influence of defects, such as dislocations, on phase stability and phase transitions. Our scattering program, combined with theoretical modeling, will address these issues in a comprehensive way using synchrotron X-rays and reactor-based neutrons. This research will be conducted with students who will receive training in one of the contemporary forefront areas of condensed matter physics. Through this they will be prepared to enter the scientific/technological workforce of the next few decades.